Abstract

Lightning, often a spectacular accompaniment to explosive volcanic eruptions, indicates the presence of large electric potential gradients resulting from the separation of electric charge over considerable distances. During magma fragmentation, initial charge separation occurs as a consequence of brittle fracture generating large areas of new surface, and releasing free electrons and ions via fractoemission. Brittle fracture may result from high strain-rates due to rapid gas-phase expansion and high energy collisions between pyroclasts, with both processes acting to reduce grain size. Charge is then further separated by fluid dynamics and scavenging of ions and electrons by surfaces. The common presence of lightning at the vent indicates that charge separation processes are operating within the conduit at magmatic temperatures. Pyroclast collision within the jet and plume will continue the fractoemission process as temperature falls. The volcanic environment created is one of electrically charged ash particles in the presence of condensing and hygroscopically stabilised liquids and a gas phase with a high ion concentration. This is an environment with many similarities to that found during wet electroscrubbing of flue gases in industrial applications, and aggregation of particles is almost inevitable. Ash aggregation has great significance for the deposition of pyroclasts smaller than c. 1 mm because sedimentation rates can increase by orders of magnitude compared to sedimentation of non-aggregated ash. Here, we discuss the ubiquitous nature of electrically charged ash surfaces as a fundamental outcome of fragmentation physics. The resultant aggregation generates a range of aggregate types that significantly changes the nature of both the atmospheric distribution of ash in time and space, and the nature of the deposits on the ground. Developing understanding of all ash aggregation processes as plume conditions change from being volcanically dominated to atmosphere controlled is vital to the accurate forecasting of volcanic ash in the atmosphere, as well as interpretation of ash fall deposits.